In the case of manufacturing a multilayer printed wiring board by e.g. a build-up method, it is necessary to flatten the surface of a lower layer substrate in order to increase the wiring density. However, the circuit pattern of a printed board, which is manufactured via a subtractive method of removing unnecessary parts of copper foil through etching, is generally formed into an uneven state in which the circuit pattern is raised from the surface of the base material.
Thus, a method has been proposed in order to flatten the printed board, of which the surface is formed in such an uneven form, in which a thermosetting resin is filled into the spaces between the circuit patterns and cured by heating. The surface of the resin is then polished.
The above described method for filling the spaces between circuit patterns with a resin includes for example, a method for applying a resin via screen printing and a method for laminating a semi-cured resin sheet. However, the method of applying a resin via screen printing has a problem in that air is inevitably involved with the resin during the printing. The involved air results in minute air bubbles, causing voids to be produced in the resin layer. The method of laminating a resin sheet also has a problem that air enters between the resin sheet and the substrate and results in air bubbles. This also causes voids to be produced in the resin layer. Such voids are not preferred because they tend to burst during the heating stage of a subsequent process, or the voids may deteriorate the electrical characteristics of the substrate.
In recent years, there has been proposed a technique for pressing the resin layer in a reduced pressure atmosphere as a method for removing the air bubbles in the resin, which cause the voids. The air bubbles in the resin are pressed in a reduced pressure atmosphere so that the air bubbles are moved to the surface of the resin layer and then released to the outside. In this case, a resin with a lower viscosity facilitates the movement of the air bubbles in the resin, so as to allow the air bubbles to be more effectively removed. On the other hand, however, there is a problem that a resin with a lowered viscosity can flow out from the spaces between the circuit patterns when the resin layer is pressed.
The present invention has been made in view of the above described circumstances. An object of the present invention is to provide a method for manufacturing a printed wiring board that makes it possible to sufficiently remove air bubbles in the resin and to provide a generally excellent curing state of the resin.